Chlamydia trachomatis is the leading cause of sexually transmitted diseases (STD) in the developed world. The majority of these infections are subclinical and non-acute in presentation which accounts for the large number of women with tubal infertility and ectopic pregnancies that are so prevalent today. Further, up to 50% of adolescent and young adult women become reinfected with Chlamydia which contributes to tubal scarring. Thus, adolescents are an important target group in which to analyze transmission patterns and types of recurrence in order to design appropriate interventional strategies. Strain typing is essential for defining the molecular epidemiology of chlamydial infections within the context of different chlamydial asymptomatic and symptomatic disease states that include urethritis, cervicitis, pelvic inflammatory disease, and recurrent infection. Monoclonal antibodies to the major outer membrane protein (MOMP) of C. trachomatis have historically been used for typing. Yet, serotypes do not reflect the same degree of variation found at the gene and protein level of this organism. ompl genotyping has been developed which is based on the MOMP gene (ompl ). It is capable of detecting new subtypes of currently known serotypes and provides a powerful alternative to immunotyping. Genotyping is an appropriate tool for evaluating transmission patterns and recurrent infections, and for use as a biologic measure of behavior in behavioral related studies and for public health surveillance. Specific Aims: 1) define the ompl genotypes and variants among adolescent males and females who differ by force-of-infection and behavioral variables; 2) analyze transmission patterns of ompl genotypes between partners and within teen groups; 3) evaluate types of recurrent infections that occur in adolescents in comparison with first (naive) or previous infections, define types of mutations that occur in ompl over these time points, and evaluate the role of genotype polymorphism, co-pathogens, and immune response in reinfection at cervical and urethral sites; and 4) define molecular characteristics of genotypes that may be associated with clinical and microbiologic data; analyze the sequence-structure/function relationships of MOMP from prototype and variant strains.